Question

What is the air gap flux density for PMDC motor?

(a) Bg= µ0(tm/lg) Hm

(b) Bg= – µ0Hm

(c) Bg= – µ0(tm/lg) Hm

(d) Bg= – µ0(lg/tm) Hm

The question was posed to me during an internship interview.

Query is from PMDC Motors topic in portion DC Machines of DC Machines

Answer 1

Right choice is (c) Bg= – µ0(tm/lg) Hm

The explanation: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*lg* Hg + 2*tm Hm = 0 and we know that air gap flux density is equal to µ times the magnetic field intensity.